Method of manufacturing optical fiber preform and optical fiber preform

ABSTRACT

The present invention relates to a method of manufacturing an optical fiber preform for obtaining an optical fiber with low transmission loss. A core preform included in the optical fiber preform comprises three or more core portions, which are each produced by a rod-in-collapse method, and in which both their alkali metal element concentration and chlorine concentration are independently controlled. In two or more manufacturing steps of the manufacturing steps for each of the three or more core portions, an alkali metal element is added. As a result, the mean alkali metal element concentration in the whole core preform is controlled to 7 atomic ppm or more and 70 atomic ppm or less.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of PCT/JP2015/070717claiming the benefit of priority of the Japanese Patent Application No.2014-148557 filed on Jul. 22, 2014, the entire contents of which areincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a method of manufacturing an opticalfiber preform and an optical fiber preform.

BACKGROUND ART

As an optical fiber with reduced Rayleigh scattering and lowtransmission loss, a silica glass-based optical fiber is known, in whichan alkali metal element is added to the core portion (e.g. see PatentDocuments 1 to 11). Herein, in a case where an alkali metal element isadded to the core portion of an optical fiber preform (corresponding toa core preform), when the optical fiber preform is drawn, the viscosityof the core portion can be reduced. In addition, the network structureof the silica glass is loosened due to a reduction in the viscosity ofthe core portion, and thus as the concentration of the alkali metal tobe added increases, the fictive temperature in the optical fiberdecreases. Accordingly, transmission loss in the optical fiber can bereduced.

As a method of adding an alkali metal element to the inside of silicaglass, a diffusion method is known (e.g. see Patent Documents 1 and 2).This diffusion method is a method in which while introducing the rawmaterial vapor of, for example, an alkali metal element or an alkalimetal salt, a raw material, in the inside of a glass pipe, the alkalimetal element is diffused and added to the inner surface of the glasspipe by heating the glass pipe with an external heat source, or bygenerating plasma in the glass pipe.

As described above, an alkali metal element is added adjacent to theinner surface of a glass pipe, and the diameter of this glass pipe isthen contracted by heating. After the diameter contraction, for thepurpose of removing transition metal elements such as Ni and Fesimultaneously added when adding an alkali metal element, the innersurface of the glass pipe is etched to only a certain degree ofthickness. Because alkali metal elements are diffused faster thantransition metal elements, an alkali metal element can be allowed toremain in a glass pipe even when transition metal elements are removedby etching a glass surface to only a certain degree of thickness. Afteretching, the glass pipe is collapsed by heating to manufacture an alkalimetal element-added core rod. A cladding portion with a refractive indexlower than the refractive index of a core portion comprising such alkalimetal element-added core rod is synthesized on the outer side of thealkali metal element-added core rod to manufacture an optical fiberpreform. An optical fiber can be manufactured by drawing the obtainedoptical fiber preform.

CITATION LIST Patent Literature

Patent Document 1: Japanese Unexamined Patent Publication No.2005-537210

Patent Document 2: US 2006/0130530 A

Patent Document 3: Japanese Unexamined Patent Publication No.2007-504080

Patent Document 4: Japanese Unexamined Patent Publication No.2008-536190

Patent Document 5: Japanese Unexamined Patent Publication No.2010-501894

Patent Document 6: Japanese Unexamined Patent Publication No.2009-541796

Patent Document 7: Japanese Unexamined Patent Publication No.2010-526749

Patent Document 8: WO 98/002389 A

Patent Document 9: U.S. Pat. No. 5,146,534

Patent Document 10: Japanese Unexamined Patent Publication No.2009-190917

Patent Document e 11: Japanese Unexamined Patent Publication No.2012-229150

SUMMARY OF INVENTION Technical Problem

As a result of investigations on the conventional techniques asdescribed above, the present inventors found the following problems. Acore rod forming the whole or part of a core preform is produced bydiffusing and adding an alkali metal element to the inner surface of aglass pipe by the diffusion method as described above and collapsingthis glass pipe by heating. Because the core rod thus produced becomesthe core portion of an optical fiber after drawing or a part of the coreportion, the alkali metal element concentration is high in the coreportion center (that is, the fictive temperature is low), while thealkali metal element concentration gradually decreases toward the outerside (that is, the fictive temperature becomes higher). Considering thedistribution of optical power propagated into the core portion, thefictive temperature in the outer peripheral portion of the core portionwith a high optical power cannot be sufficiently reduced due to suchdistribution of alkali metal element concentration, and thustransmission loss is not reduced. On the other hand, even when a highconcentration of alkali metal element is added to a large portion of thecore portion of an optical fiber preform at one time for the purpose ofreducing transmission loss, crystallization occurs in the core portion.This has caused problems in that the transmission loss of an opticalfiber obtained by drawing increases or the yield rate of an opticalfiber decreases.

The present invention is made to solve the problems as described above,and an object thereof is to provide an optical fiber preform, from whichan optical fiber with low transmission loss can be manufactured bydrawing, and a method by which such optical fiber preform can bemanufactured.

Solution to Problem

The method of manufacturing an optical fiber preform according to thepresent embodiment is a method of manufacturing an optical fiberpreform, which comprises a core portion having a first core portioncomprising an alkali metal element, a second core portion surroundingthe first core portion, and a third core portion surrounding the secondcore portion, and a cladding portion surrounding the core portion andhaving a refractive index lower than the refractive index of such coreportion. In the description, the core portion having the first coreportion, the second core portion and the third core portion is describedas a “core preform.” In addition, such method of manufacturing anoptical fiber preform comprises at least a first doping step, a firstcollapse step, a first diameter-reduction step, a second doping step, asecond collapse step, a first build-up step and a second build-up stepto solve the problems as described above. In the first doping step, thefirst doping of an alkali metal element to the inner surface of a firstglass pipe is carried out. In the first collapse step, a firstintermediate rod is produced from the first glass pipe. In the firstdiameter-reduction step, a first core rod forming a part of a first coreportion is produced from the first intermediate rod. In the seconddoping step, the second doping of an alkali metal element to the innersurface of a second glass pipe is carried out. In the second collapsestep, a second intermediate rod comprising a first glass region to bethe first core portion and a second glass region to be the second coreportion is produced from the first core rod and the second glass pipe.In the first build-up step, a third intermediate rod comprising thesecond core rod formed from the whole or part of the second intermediaterod is obtained. In the second build-up step, an optical fiber preformcomprising the third core rod formed from the whole or part of the thirdintermediate rod is obtained.

Advantageous Effects of Invention

According to the present embodiment, an optical fiber with lowtransmission loss can be obtained by drawing the produced optical fiberpreform.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flow chart illustrating the method of manufacturing anoptical fiber preform and an optical fiber according to the presentembodiment.

FIG. 2 is an example of the profile of potassium concentration(K-concentration) of a second intermediate rod produced in the secondcollapse step S10 of the method of manufacturing an optical fiberpreform according to the present embodiment (a glass rod produced in thesecond collapse step S10).

FIG. 3 is an example of the profile of potassium concentration(K-concentration) of a first intermediate rod produced in the firstcollapse step S25 of the method of manufacturing an optical fiberpreform according to Comparative Example (a glass rod produced in thefirst collapse step S25).

FIG. 4 is about a plurality of optical fiber preform samples accordingto the present embodiment and a plurality of optical fiber preformsamples according to Comparative Example, and a table summarizing themean potassium concentration (mean K-concentration) in a core preform(which forms a part of an optical fiber preform), the potassiumconcentration peak (K-concentration peak) in such core preform, and thetransmission loss of an optical fiber at a wavelength of 1550 nm.

FIG. 5 is about a plurality of optical fiber preform samples accordingto the present embodiment and a plurality of optical fiber preformsamples according to Comparative Example, and a graph showing arelationship of the potassium concentration peak (K-concentration peak)in a core preform (which forms a part of an optical fiber preform), andthe transmission loss of an optical fiber at a wavelength of 1550 nm.

FIG. 6 is about a plurality of optical fiber preform samples accordingto the present embodiment and a plurality of optical fiber preformsamples according to Comparative Example, and a graph showing arelationship of the mean potassium concentration (mean K-concentration)in a core preform (which forms a part of an optical fiber preform), andthe potassium concentration peak (K-concentration peak) in such corepreform.

FIG. 7 is a cross-sectional view of an optical fiber preform accordingto the present embodiment.

FIG. 8 is the profile of potassium concentration (K-concentration) alongthe radial direction in a core preform, especially a first core portion.

DESCRIPTION OF EMBODIMENTS Description of Embodiment in Invention ofApplication

First, the present embodiment in the invention of the application willbe listed and described.

(1) The method of manufacturing an optical fiber preform according tothe present embodiment is a method of manufacturing an optical fiberpreform, which comprises a first core portion comprising an alkali metalelement, a second core portion surrounding the first core portion, and athird core portion surrounding the second core portion, a claddingportion surrounding the third core portion and further having arefractive index lower than each refractive index of the first to thirdcore portions, and a core preform is formed from the first to third coreportions. Such method of manufacturing an optical fiber preformcomprises at least a first doping step, a first collapse step, a firstdiameter-reduction step, a second doping step, a second collapse step, afirst build-up step and a second build-up step. In the first dopingstep, an alkali metal element is added to the inner surface of a firstglass pipe with a mean chlorine concentration of 10 atomic ppm or moreand 600 atomic ppm or less. In the first collapse step, the first glasspipe after the first doping step is collapsed by heating. By this firstcollapse step, a first intermediate rod is produced from the first glasspipe. In the first diameter-reduction step, the diameter of the firstintermediate rod is reduced by removing the outer peripheral portion ofthe first intermediate rod. By this first diameter-reduction step, afirst core rod forming a part of the first core portion is produced formthe first intermediate rod. In the second doping step, an alkali metalelement is added to the inner surface of a second glass pipe with themean chlorine concentration of 10 atomic ppm or more and 600 atomic ppmor less. In the second collapse step, the first core rod and the secondglass pipe are unified by heating with the first core rod inserted intothe second glass pipe after the second doping step. By this secondcollapse step, a second intermediate rod comprising a first glass regionto be the first core portion and a second glass region to be the secondcore portion is produced. In this second intermediate rod, the firstglass region has an alkali metal element concentration of 100 atomic ppmor more. In addition, the second glass region has an alkali metalelement concentration of 10 atomic ppm or less. In the first build-upstep, a third glass region to be the third core portion is added ontothe outer periphery of the second core rod formed from the whole or partof the second intermediate rod. This third glass region has an alkalimetal element concentration of 10 atomic ppm or less and a chlorineconcentration of 2,000 atomic ppm or more and 15,000 atomic ppm or less.By this first build-up step, a third intermediate rod comprising thesecond core rod is obtained. In addition, in the second build-up step, afourth glass region to be the cladding portion is added onto the outerperiphery of a third core rod formed from the whole or part of the thirdintermediate rod. By this second build-up step, an optical fiber preformcomprising the third core rod is obtained. Herein, the “atomic ppm” isone of units representing the concentration of dopants such as alkalimetals and chlorine and fluorine in glass, and means the number of atomscontained in one million molecules of SiO₂.

For the above-mentioned structure (1), all structures described belowand all combinations of these structures can be applied. That is, thecollapse of a first glass pipe in the first collapse step is suitablycarried out while introducing oxygen gas in the inside of the firstglass pipe in a reduced pressure condition. In addition, the integrationof the first core rod and the second glass pipe in the second collapsestep is suitably carried out while introducing oxygen gas in the insideof the second glass pipe in a reduced pressure condition. Such method ofmanufacturing an optical fiber preform can further comprise a seconddiameter-reduction step carried out after the second collapse step. Inthis second diameter-reduction step, the diameter of a secondintermediate rod is reduced by removing the outer peripheral portion ofthe second intermediate rod. Accordingly, a second core rod is producedfrom the second intermediate rod. In addition, such method ofmanufacturing an optical fiber preform can further comprise a thirddiameter-reduction step carried out after the first build-up step. Inthis third diameter-reduction step, the diameter of a third intermediaterod is reduced by removing the outer peripheral portion of the thirdintermediate rod. Accordingly, a third core rod is produced from thethird intermediate rod. Furthermore, the mean concentration of OH groupcontained in the third core rod is suitably 0.01 wt·ppm or less.Furthermore, an alkali metal element is suitably potassium (K). Theoxygen molecule concentration is suitably 30 mol·ppb or more and 200mol·ppb or less in a part or the whole of a region, in which the alkalimetal element concentration is 100 atomic ppm or more, of the third corerod. Herein, “wt·ppm” is one of units representing the concentration ofdopants such as OH group in glass, and means the weight [g] of dopantscontained in 1,000,000 g of SiO₂.

(2) In addition, the optical fiber preform according to the presentembodiment is obtained for example by the method of manufacturing anoptical fiber preform according to the present embodiment as describedabove (the method of manufacturing an optical fiber preform prescribedby all of the structures described above and all combinations of thesestructures). Such optical fiber preform comprises a core preform and acladding portion surrounding the core preform. The core preform isformed from at least a first core portion comprising the central axis ofsuch core preform, a second core portion surrounding the first coreportion, and a third core portion surrounding the second core portion.The first core portion has a maximum alkali metal element concentrationof 500 atomic ppm or more and 5,000 atomic ppm or less. In the firstcore portion, the distribution of alkali metal element concentrationalong the radial direction of such first core portion has a form inwhich at least two points, which are separated for a predetermineddistance from the center of the cross section of such first core portion(the intersection point of the cross section of the first core portionand the central axis), are local maxima (e.g. FIGS. 2 and 8). The secondcore portion has an alkali metal element concentration of 10 atomic ppmor less and a chlorine concentration of 10 atomic ppm or more and 600atomic ppm or less. The third core portion has an alkali metal elementconcentration of 10 atomic ppm or less and a chlorine concentration of2,000 atomic ppm or more and 15,000 atomic ppm or less. In the wholecore preform, the mean alkali metal element concentration is preferably7 atomic ppm or more and 70 atomic ppm or less. In addition, therefractive index of a cladding portion is lower than each refractiveindex of the first to third core portions.

For the above-mentioned structure (2), all structures described belowand all combinations of these structures can be applied. That is, analkali metal element is suitably potassium. The chlorine concentrationof the first core portion and the second core portion is suitably 10atomic ppm or more and 600 atomic ppm or less. The oxygen moleculeconcentration is suitably 30 mol·ppb or more and 200 mol·ppb or less ina part or the whole of a region, in which the alkali metal elementconcentration is 100 atomic ppm or more, of a core preform. Herein,“mol·ppb” is one of units representing the concentration of dopants suchas oxygen molecule in glass, and means the molecular weight [mol] ofdopants contained in 1,000,000 mol of SiO₂. The mean concentration of OHgroup in a core preform is suitably 0.01 wt·ppm or less.

Details of Embodiment in Invention of Application

The method of manufacturing an optical fiber preform and a specificstructure of the optical fiber preform according to the presentembodiment will now be described in detail with reference toaccompanying drawings. It should be noted that the present invention isnot limited to these exemplifications, defined by the appended claims,and intended to include any modifications within the meaning and rangeequivalent to the scope of the claims.

FIG. 1 is a flow chart illustrating the method of manufacturing anoptical fiber preform and an optical fiber according to the presentembodiment. The method of manufacturing an optical fiber preformaccording to the present embodiment comprises a preparation step S1, afirst doping step S2, a first diameter-contraction step S3, a firstetching step S4, a first collapse step S5, a first diameter-reductionstep S6, a second doping step S7, a second diameter-contraction step S8,a second etching step S9, a second collapse step S10, a seconddiameter-reduction step S11, a third collapse step S12, acore-elongation step S13, a third diameter-reduction step S14, a fourthcollapse step S15, an elongation step S16 and asecond-cladding-portion-addition step S17, and by carrying out thesesteps in turn, an optical fiber preform is produced. The method ofmanufacturing an optical fiber according to the present embodiment canmanufacture an optical fiber by further carrying out a drawing step S18.The method of manufacturing an optical fiber preform and the method ofmanufacturing an optical fiber will now be described by way of aspecific example of manufacturing conditions.

In the preparation step S1, a first glass pipe is prepared. The firstglass pipe is formed from silica-based glass. In the first glass pipe,the chlorine (Cl) concentration is 150 atomic ppm, the fluorine (F)concentration is 6,000 atomic ppm, and the concentration of otherdopants and impurities is 10 mol·ppm or less. In addition, this firstglass pipe has an outside diameter of 35 mm and an inside diameter of 20mm.

In the first doping step S2, an alkali metal element is added to theinner surface of the first glass pipe. Specifically, potassium bromide(KBr) is used as an alkali metal raw material and KBr vapor is generatedby heating such potassium bromide with a heat source to a temperature of840° C. While introducing KBr vapor along with 1 slm (1 litter/min inthe standard state) of oxygen introduced as a carrier gas in the insideof the first glass pipe, such first glass pipe is heated with anoxyhydrogen burner from the outside so that the surface of the firstglass pipe will be 2150° C. The heating in this first doping step S2 iscarried out while traversing the oxyhydrogen burner at a velocity of 40m/min for a total of 15 turns, and by this heating, the potassiumelement is diffused into the inner surface of the first glass pipe.

In the first diameter-contraction step S3, the diameter of the firstglass pipe is reduced by heating. Specifically, while sending oxygen(0.5 slm) in the inside of the first glass pipe to which potassiumelement has been added, such first glass pipe is heated with anoxyhydrogen burner so that the outer surface of the first glass pipewill be 2250° C. The heating in this first diameter-contraction step S3is carried out while traversing an oxyhydrogen burner a plurality oftimes, and the diameter of the first glass pipe is contracted until theinside diameter is 5 mm.

In the first etching step S4, the inner surface of the first glass pipeis etched to remove transition metal elements such as Ni and Fe and OHgroup simultaneously added when adding an alkali metal element in thefirst doping step S2. Specifically, the etching to the inner surface ofthe first glass pipe is gas-phase etching, which is carried out byheating the first glass pipe with an oxyhydrogen burner whileintroducing a mixed gas of SF₆ (0.2 slm) and oxygen (0.5 slm) in theinside of the first glass pipe to which potassium element has beenadded.

In the first collapse step S5, the first glass pipe is collapsed toproduce a first intermediate rod (a glass rod produced in the firstcollapse step S5). Specifically, oxygen (2 slm) is introduced in theinside of the first glass pipe with the inside of the first glass pipeafter the first etching step S4 being under reduced pressure with anabsolute pressure of 97 kPa or less. The first glass pipe is collapsedby heating the surface of the first glass pipe to a temperature of 2150°C. with an oxyhydrogen burner in parallel with the oxygen introductionunder reduced pressure described above. By this means, a firstintermediate rod with a diameter of 25 mm, to which potassium elementhas been added, is produced.

In the first diameter-reduction step S6, a first core rod with a reduceddiameter (a first glass rod manufactured in the first diameter-reductionstep S6) is produced by removing the outer peripheral portion of thefirst intermediate rod produced in the first collapse step S5.Specifically, the first core rod is obtained by hollowing out the centerportion with a diameter of 5 mm of the first intermediate rod producedin the first collapse step S5 by boring. The first core rod can be alsoobtained by removing the outer peripheral portion of the firstintermediate rod produced in the first collapse step S5 with the centerportion left by grinding. The potassium concentration in the surfaceportion of the first core rod produced herein is 100 atomic ppm.

In the second doping step S7, potassium element is added to the innersurface of a second glass pipe. The second glass pipe is formed from thesame silica-based glass as of the first glass pipe (the refractiveindices of the first and second glass pipes are almost identical).Potassium element is added to the second glass pipe in the same manneras in the first doping step S2.

In the second diameter-contraction step S8, the diameter of the secondglass pipe is contracted by heating. Specifically, while sending oxygen(0.5 slm) in the inside of the second glass pipe to which potassiumelement has been added, such second glass pipe is heated with anoxyhydrogen burner so that the outer surface of the second glass pipewill be 2250° C. This heating in the second diameter-contraction step S8is carried out while traversing the oxyhydrogen burner 6 times. Theinside diameter of the second glass pipe after the diameter contractionis about 0.1 to 1 mm larger than the outside diameter of the first corerod produced in the first diameter-reduction step S6.

In the second etching step S9, the inner surface of the second glasspipe is etched to remove transition metal elements such as Ni and Fe andOH group simultaneously added when adding an alkali metal element in thesecond doping step S7. Specifically, the etching to the inner surface ofthe second glass pipe is gas-phase etching, which is carried out byheating the second glass pipe with an oxyhydrogen burner whileintroducing a mixed gas of SF₆ (0.2 slm) and oxygen (0.5 slm) in theinside of the second glass pipe to which potassium element has beenadded.

In the second collapse step S10, the first core rod manufactured in thefirst diameter-reduction step S6 is inserted in the inside of the secondglass pipe after the second etching step S9. A second intermediate rod(a glass rod produced in the second collapse step S10) is then producedby the rod-in-collapse method of unifying the first core rod and thesecond glass pipe by heating. In the second collapse step S10,specifically, oxygen (2 slm) is introduced in the inside of the secondglass pipe with the inside of the second glass pipe being under reducedpressure with an absolute pressure of 97 kPa or less in the same manneras in the first collapse step S5. The rod-in-collapse (the integrationof the first core rod and the second glass pipe) is carried out byheating the surface of the second glass pipe to a temperature of 2150°C. with an oxyhydrogen burner in parallel with the oxygen introductionunder reduced pressure described above.

In the second diameter-reduction step S11, the outer peripheral portionof the second intermediate rod produced in the second collapse step S10is removed to produce a second core rod (a second glass rod produced inthe second diameter-reduction step S11). The outer peripheral portion ofthe second intermediate rod can be removed by mechanical or chemicalgrinding to such outer peripheral portion. In addition, such outerperipheral portion can be removed by physical separation from the centerportion of the second intermediate rod (a portion to be a second corerod), which is hollowed out by boring. The diameter of the second corerod produced herein is 16 mm. In addition, the second core rod is not ina state in which potassium element is added as a whole, and potassiumelement is not purposely added at least in the outer peripheral regionof the second core rod. That is, this second core rod has a first coreportion (a first glass region located in the inner part of the rod) anda second core portion surrounding the first core portion (a second glassregion located in the outer part of the rod), and the first core portionhas a chlorine concentration of 150 atomic ppm and a fluorineconcentration of 6,000 atomic ppm, and further comprises potassiumelement. On the other hand, the second core portion has a chlorineconcentration of 150 atomic ppm and a fluorine concentration of 6,000atomic ppm, while the potassium element concentration in such secondcore portion is 10 atomic ppm or less and the second core portion doesnot substantially comprise potassium element. Therefore, the refractiveindex of the first core portion and the refractive index of the secondcore portion are almost identical. The profile of potassium elementconcentration in the first core portion has the first concentration peak(the local maximal value) caused by the first doping step around thecentral axis and further a second concentration peak caused by thesecond doping step on the circumference surrounding the first peak.

In the third collapse step S12, a third core portion (a third glassregion) is added onto the outer periphery of the second core rod. Inthis step, specifically, a third glass pipe formed from silica-basedglass which has a chlorine concentration of 12,000 atomic ppm and doesnot substantially comprise dopants other than chlorine (having arefractive index higher than the refractive indices of the first andsecond glass pipes) is prepared. With the second core rod inserted intothe prepared third glass pipe, the third glass pipe and the second corerod are unified by heating. By such rod-in-collapse method, the thirdcore portion is added onto the outer periphery of the second core rod toproduce a third intermediate rod (a third glass rod produced in thethird collapse step S12). This third intermediate rod is a portion to bethe core portion of an optical fiber.

In the core-elongation step S13, the third intermediate rod produced inthe third collapse step S12 is elongated while being heated.Accordingly, the outside diameter of the third intermediate rod is 27mm.

In the third diameter-reduction step S14, the outer peripheral portionof the third intermediate rod elongated in the core-elongation step S13is removed to produce a third core rod with a diameter of 20 mm (a corerod produced in the third diameter-reduction step S14), i.e. a corepreform. The outer peripheral portion of the third intermediate rod canbe removed by mechanical or chemical grinding to such outer peripheralportion. In addition, such outer peripheral portion can be removed byphysical separation from the center portion of the third intermediaterod (a portion to be a third core rod), which is hollowed out by boring.

This core preform (the core rod produced in the third diameter-reductionstep S14) has a first core portion, a second core portion surroundingthe first core portion, and a third core portion surrounding the secondcore portion. The first core portion comprises potassium element. Inaddition, in this first core portion, the chlorine concentration is 150atomic ppm and the fluorine concentration is 6,000 atomic ppm. In thesecond core portion, the chlorine concentration is 150 atomic ppm, thefluorine concentration is 6,000 atomic ppm, and the potassium elementconcentration is 10 atomic ppm or less. In the third core portion, thechlorine concentration is 12,000 atomic ppm, and the potassiumconcentration is 10 atomic ppm or less. The second core portion and thethird core portion do not substantially comprise potassium element. Theratio of the diameter of the first core portion forming a part of thecore preform and the diameter of the core preform (20 mm) is 5 times.

The mean concentration of OH group contained in the core preform is 0.01wt·ppm or less. When an optical fiber is manufactured by drawing anoptical fiber preform comprising a core preform by a known method, anincrease in transmission loss due to OH group absorption in the 1.38 μmwavelength band is less than 1 dB/km. In addition, the meanconcentration of OH group is further preferably 0.001 wt·ppm or less. Atthis time, an increase in transmission loss due to OH group absorptionin the 1.38 μm wavelength band is less than 0.1 dB/km in themanufactured optical fiber.

In the fourth collapse step S15, a first cladding portion is added ontothe outer periphery of the third core portion (corresponding to thethird core rod as a core preform). In this step, specifically, a fourthglass pipe formed from silica-based glass to which fluorine has beenadded (having a refractive index lower than the refractive indices ofthe first to second glass pipes) is prepared. With the core preforminserted into this fourth glass pipe, the fourth glass pipe and the corepreform are unified by heating. By such rod-in-collapse method, thefirst cladding portion is added onto the outer periphery of the thirdcore portion. A relative index difference between the core preformcomprising the first to third core portions and the first claddingportion is up to about 0.34%.

In the elongation step S16, a fourth intermediate rod obtained by theintegration of the core preform and the fourth glass pipe in the fourthcollapse step S15 (a glass rod produced in the fourth collapse step S15)is elongated while being heated. This elongating of the fourthintermediate rod is for the adjustment of the diameter of such fourthintermediate rod to obtain a desired value of the core portion diameterof an optical fiber manufactured in the drawing step S18.

In the second-cladding-portion-addition step S17, a second claddingportion is added onto the outer periphery of the first cladding portion.In this step, specifically, the second cladding portion formed fromsilica-based glass to which fluorine has been added is synthesized ontothe outer periphery of the fourth intermediate rod after the elongationstep S16 for example by the OVD method, VAD method or rod-in-collapsemethod to manufacture an optical fiber preform. In an optical fiberpreform manufactured through the above steps, the refractive index ofthe first core portion and the refractive index of the second coreportion are almost identical, the refractive index of the third coreportion is higher than the refractive indices of the first and secondcore portions, and the refractive indices of the first cladding portionand the second cladding portion are lower than each refractive index ofthe first to third core portions.

In the drawing step S18, a desired optical fiber is manufactured bydrawing the optical fiber preform manufactured through the above steps.

FIG. 2 is an example of the profile of potassium concentration of thesecond intermediate rod produced in the second collapse step S10 of themethod of manufacturing an optical fiber preform according to thepresent embodiment. In this example, the potassium concentration peak is1,390 atomic ppm. In a core preform forming a part of the manufacturedoptical fiber preform, the mean potassium concentration is 18 atomicppm, and the potassium concentration peak is 1,390 atomic ppm. Thetransmission loss of the manufactured optical fiber (the optical fiberafter drawing) at a wavelength of 1550 nm is 0.150 dB/km.

In the method of manufacturing an optical fiber preform according to thepresent embodiment described above, potassium element is added twice,and thus the manufacturing method according to the present embodimentwill now be described as a “twice K-doping method.” In a method ofmanufacturing an optical fiber preform in Comparative Example describedbelow, by contrast, potassium is added only once, and thus themanufacturing method according to Comparative Example will now bedescribed as “one time K-doping method.”

The method of manufacturing an optical fiber preform in ComparativeExample comprises a preparation step S21 (corresponding to thepreparation step S1 in FIG. 1), a first doping step S22 (correspondingto the first doping step S2 in FIG. 1), a first diameter-contractionstep S23 (corresponding to the first diameter-contraction step S3 inFIG. 1), a first etching step S24 (corresponding to the first etchingstep S4 in FIG. 1), a first collapse step S25 (corresponding to thefirst collapse step S5 in FIG. 1), a first grinding step S26(corresponding to the second diameter-reduction step S11 in FIG. 1), asecond collapse step S27 (corresponding to the third collapse step S12in FIG. 1), a core-elongation step S28 (corresponding to thecore-elongation step S13 in FIG. 1), a second grinding step S29(corresponding to the third diameter-reduction step S14 in FIG. 1), athird collapse step S30 (corresponding to the fourth collapse step S15in FIG. 1), an elongation step S31 (corresponding to the elongation stepS16 in FIG. 1) and a second-cladding-portion-addition step S32(corresponding to the second-cladding-portion-addition step S17 in FIG.1), and by carrying out these steps in turn, an optical fiber preform ismanufactured. In the method of manufacturing an optical fiber inComparative Example, an optical fiber can be manufactured by furthercarrying out a drawing step S33 (corresponding to the drawing step S18in FIG. 1). The method of manufacturing an optical fiber preform and themethod of manufacturing an optical fiber will now be described with aspecific example of manufacturing conditions.

In Comparative Example, the preparation step S21, the first doping stepS22, the first diameter-contraction step S23, the first etching step S24and the first collapse step S25 are the same as the preparation step S1,the first doping step S2, the first diameter-contraction step S3, thefirst etching step S4 and the first collapse step S5 in the presentembodiment (FIG. 1), respectively.

In the first grinding step S26, the outer peripheral portion of a firstintermediate rod (a glass rod) produced in the first collapse step S25is ground to produce a first core rod (a first glass rod). The diameterof the first core rod produced herein is 16 mm. In addition, the firstcore rod is not in a state in which potassium element is added as awhole, and potassium element is not purposely added at least in theouter peripheral region of the first core rod. That is, this first corerod has a first core portion (a first glass region located in the innerside of the rod) and a second core portion surrounding the first coreportion (a second glass region located in the outer side of the rod),and the first core portion has a chlorine concentration of 150 atomicppm and a fluorine concentration of 6,000 atomic ppm, and furthercomprises potassium element. On the other hand, the second core portionhas a chlorine concentration of 150 atomic ppm and a fluorineconcentration of 6,000 atomic ppm, while the potassium elementconcentration in such second core portion is 10 atomic ppm or less andthe second core portion does not substantially comprise potassiumelement.

In the second collapse step S27, a third core portion is added onto theouter periphery of the first core rod. In this step, specifically, asecond glass pipe formed from silica-based glass which has a chlorineconcentration of 13,000 atomic ppm and does not substantially comprisedopants other than chlorine is prepared. With the first core rodinserted into this prepared second glass pipe, the second glass pipe andthe first core rod are unified by heating. By such rod-in-collapsemethod, the third core portion is added onto the outer periphery of thefirst core rod to produce a second intermediate rod. This secondintermediate rod is a portion to be a core preform of an optical fiber.

In the core-elongation step S28, the second intermediate rod produced inthe second collapse step S27 is elongated while being heated.Accordingly, the outside diameter of the second intermediate rod is 27mm.

In the second grinding step S29, the outer peripheral portion of thesecond intermediate rod elongated in the core-elongation step S28 isground to produce a core preform with a diameter of 20 mm.

This core preform has the first core portion comprising potassiumelement, the second core portion surrounding the first core portion, andthe third core portion surrounding the second core portion. In the firstcore portion comprising potassium element, the chlorine concentration is150 atomic ppm, and the fluorine concentration is 6,000 atomic ppm. Inthe second core portion, the chlorine concentration is 150 atomic ppm,the fluorine concentration is 6,000 atomic ppm, and the potassiumelement concentration is 10 atomic ppm or less. In the third coreportion, the chlorine concentration is 12,000 atomic ppm, and thepotassium concentration is 10 atomic ppm or less. As described above,the second core portion and the third core portion do not substantiallycomprise potassium element. The ratio of the diameter of the first coreportion forming a part of a core preform and the diameter of the corepreform (20 mm) is 5 times.

In the third collapse step S30, a first cladding portion is added ontothe outer periphery of the third core portion. In this step,specifically, a third glass pipe formed from silica-based glass to whichfluorine has been added is prepared. With the core preform inserted intothis prepared third glass pipe, the third glass pipe and the corepreform are unified by heating. By such rod-in-collapse method, a firstcladding portion is added onto the outer periphery of the third coreportion. A relative index difference between the core preform comprisingthe first to third core portions and the first cladding portion is up toabout 0.34%.

In the elongation step S31, a third intermediate rod obtained by theintegration of the core preform and the third glass pipe in the thirdcollapse step S30, is elongated while being heated. This elongating ofthe third intermediate rod is for the adjustment of the diameter of suchthird intermediate rod to obtain a desired value of the core portiondiameter of an optical fiber manufactured in the drawing step S33.

In the second-cladding-portion-addition step S32, a second claddingportion is added onto the outer periphery of the first cladding portion.In this step, specifically, a second cladding portion formed fromsilica-based glass to which fluorine has been added is synthesized onthe outer periphery of the third intermediate rod after the elongationstep S31 for example by the OVD method, VAD method, or rod-in-collapsemethod to manufacture an optical fiber preform.

In the drawing step S33, the optical fiber preform manufactured throughthe above steps is drawn to manufacture an optical fiber.

FIG. 3 is an example of the profile of potassium concentration of thefirst intermediate rod produced in the first collapse step S25 of themethod of manufacturing an optical fiber preform in Comparative Example.In the example shown in FIG. 3, the potassium concentration peak is1,250 atomic ppm. In a core preform forming a part of the manufacturedoptical fiber preform, the mean potassium concentration is 8 atomic ppm,and the potassium concentration peak is 830 atomic ppm. The transmissionloss of the manufactured optical fiber at a wavelength of 1550 nm is0.154 dB/km.

By each of the method of manufacturing an optical fiber preformaccording to the present embodiment (twice K-doping method) and themethod of manufacturing an optical fiber preform in Comparative Example(one time K-doping method) as described above, the mean potassiumconcentration in a core preform forming a part of an optical fiberpreform is set to various values, and a plurality of optical fiberpreform samples is manufactured. The measurement values of the potassiumconcentration peak in each core preform of a plurality of themanufactured samples and the transmission loss of an optical fiber at awavelength of 1550 nm are shown in FIGS. 4 to 6.

FIG. 4 is about a plurality of optical fiber preform samplesmanufactured by the twice K-doping method (the present embodiment) and aplurality of optical fiber preform samples manufactured by the one timeK-doping method (Comparative Example), and a table summarizing the meanpotassium concentration in a core preform (which forms a part of anoptical fiber preform), the potassium concentration peak in such corepreform and the transmission loss of an optical fiber at a wavelength of1550 μm. In this table, the “*” mark indicates that it was difficult toobtain a fiber because glass crystallization often occurred in a corepreform.

FIG. 5 is about a plurality of optical fiber preform samplesmanufactured by the twice K-doping method (the present embodiment) and aplurality of optical fiber preform samples manufactured by the one timeK-doping method (Comparative Example), and a graph showing arelationship between the potassium concentration peak in a core preformand the transmission loss of an optical fiber at a wavelength of 1550nm. FIG. 6 is about a plurality of optical fiber preform samplesmanufactured by the twice K-doping method (the present embodiment) and aplurality of optical fiber preform samples manufactured by the one timeK-doping method (Comparative Example), and a graph showing arelationship between the mean potassium concentration in a core preformand the potassium concentration peak in the core preform. In each ofFIGS. 5 and 6, the “” mark indicates the examples of the optical fiberpreform manufactured by the twice K-doping method of the presentembodiment, and the “⋄” mark indicates the examples of the optical fiberpreform manufactured by the one time K-doping method of ComparativeExample.

As can be seen from these FIGS. 4 to 6, when the potassium concentrationpeak of a core preform is high, transmission loss can be reduced. As canbe seen from the comparison of a plurality of optical fiber preformsamples according to the present embodiment and a plurality of opticalfiber preform samples according to Comparative Example, even when thepotassium concentration peaks are identical, the transmission loss of adrawn optical fiber can be reduced by manufacturing an optical fiberpreform by the twice K-doping method of the present embodiment.

When the potassium concentration peak in a core preform is above 5,000atomic ppm, crystals are easily generated in the core preform, and thusdrawing is difficult. According to the present embodiment, the potassiumconcentration peak in a core preform is kept to 5,000 atomic ppm orless, and simultaneously the mean potassium concentration in the wholecore preform can be high compared to that of Comparative Example.

When the chlorine concentration of first and second glass pipes (both towhich an alkali metal element has been added) used in the twice K-dopingmethod in the present embodiment, and a first glass pipe (to which analkali metal element has been added) used in the one time K-dopingmethod in Comparative Example is less than 10 atomic ppm, thetransmission loss of an optical fiber manufactured using these glasspipes increases. It is thought that this is because glass defects oftenoccur in the step of drawing an optical fiber preform. On the otherhand, when the chlorine concentration is 600 atomic ppm or more in theseglass pipes, the frequency of defective products increases. It isthought this is because, when manufacturing an optical fiber preform, analkali metal element and chlorine element react to easily generate achloride, which causes crystals. Therefore, the chlorine concentrationin these glass pipes is desirably 10 atomic ppm or more and 600 atomicppm or less, and more desirably 30 atomic ppm or more and 400 atomic ppmor less. When the mean chlorine concentration in a first glass pipe anda second glass pipe is within the above-mentioned suitable range, theycan be used in the present embodiment.

As the third collapse step S12 in the method of manufacturing an opticalfiber preform according to the present embodiment (twice K-dopingmethod) and the second collapse step S27 in the method of manufacturingan optical fiber preform according to Comparative Example (one timeK-doping method), it is preferred that a third core portion formed fromsilica glass with a mean chlorine concentration of 2,000 atomic ppm ormore and 15,000 atomic ppm or less be added to the outer side of a firstcore portion to which an alkali metal element has been added. By addinga third core portion as described above, the transmission loss of adrawn optical fiber can be reduced. It is thought that this is becauseglass defects, which occur during drawing and occur in an alkalimetal-added glass region, are repaired by chlorine. However, when thechlorine concentration is too high, a chloride causing crystallizationis generated during the process of manufacturing an optical fiberpreform after adding a glass region to which a high concentration ofchlorine has been added. Therefore, the chlorine concentration of athird core portion is desirably 15,000 atomic ppm or less and moredesirably 5,000 atomic ppm or more and 14,000 atomic ppm or less.

In the core preform forming a part of an optical fiber preform accordingto the present embodiment, a second core portion having a chlorineconcentration of 10 atomic ppm or more and 600 atomic ppm or less and analkali metal concentration of 10 atomic ppm or less is preferablyprovided between a first core portion having a chlorine concentration of10 atomic ppm or more and 600 atomic ppm or less and comprising analkali metal element and a third core portion having a chlorineconcentration of 2,000 atomic ppm or more and 15,000 atomic ppm or lessand an alkali metal element of 10 atomic ppm or less. The second coreportion and the third core portion do not substantially comprise analkali metal element. This is to prevent a case in which, when an alkalimetal element which has been added to a first core portion, is diffusedinto a third core portion with a high chlorine concentration, the alkalimetal element becomes a chloride, which becomes a crystal core portion,in the heating step such as the drawing step carried out after the stepof manufacturing a core preform.

Therefore, the optical fiber preform 1 according to the presentembodiment is an optical fiber preform which comprises a core preform 10to be a core portion region of an optical fiber and a cladding portion20 to be a cladding portion region of an optical fiber as shown in thecross-sectional view in FIG. 7 and is formed from silica-based glass,wherein the optical fiber preform has the features as described below.That is, the core preform 10 has, in turn from the central axis AX alongthe radial direction r, at least a first core portion 11 comprising suchcentral axis AX (the center region of the core preform 10), a secondcore portion 12 in contact with the outer side of the first core portion11 (the intermediate region of the core preform 10), and a third coreportion 13 in contact with the outer side of the second core portion 12(the peripheral region of the core preform 10). In the first coreportion 11, the peak alkali metal element concentration is 500 atomicppm or more and 5,000 atomic ppm or less, and the chlorine concentrationis 10 atomic ppm or more and 600 atomic ppm or less. In the second coreportion 12, the alkali metal element concentration is 10 atomic ppm orless, and the chlorine concentration is 10 or more and 600 atomic ppm orless. In the third core portion 13, the alkali metal elementconcentration is 10 atomic ppm or less, and the chlorine concentrationis 2,000 atomic ppm or more. That is, the refractive index of the firstcore portion 11 and the refractive index of the second core portion 12are almost identical, and the refractive index of the third core portion13 is higher than the refractive indices of the first and second coreportions 11 and 12. In addition, fluorine is added to a cladding portion20, and the refractive index is set to be lower than each refractiveindex of the first to third core portions 11 to 13.

FIG. 8 shows an example of the profile of alkali metal elementconcentration in the core preform 10 included in the optical fiberpreform 1 shown in FIG. 7. Specifically, the profile of theconcentration in FIG. 8 is the profile of K-concentration along theradial direction r of such core preform 10 (the direction from thecentral axis AX to the outer peripheral surface of such core preform 10in the cross section of such core preform 10). In the presentembodiment, the doping of potassium element (K-doping) is carried out inthe first doping step S2 and the second doping step S7 as shown in FIG.1, and thus the profile of the concentration in the first core portion11 has a form in which two points, one which is adjacent to the centralaxis AX and is separated for a distance a from the center of the crosssection (the intersection point of the cross section of the first coreportion and the central axis AX), and the peripheral portion of thefirst core portion 11 which is separated for a distance b, are localmaxima.

In the first collapse step S5 and the second collapse step S10 in themethod of manufacturing an optical fiber preform according to thepresent embodiment, a glass pipe having an alkali metal element added tothe inner surface is preferably subjected to collapse or rod-in-collapsein an oxygen atmosphere. It is supposed that this is because many pointdefects typified by oxygen deficient center (ODC, ≡Si—Si≡) remain inglass in an atmosphere with a little oxygen, which cause an increase intransmission loss of an optical fiber. Therefore, glass defects easilyoccur especially in a glass portion to which an alkali metal element hasbeen added, and thus it is preferred that the concentration of oxygenmolecule contained in glass be 30 mol·ppb or more in a part or the wholeof a glass region, in which the alkali metal concentration is 100 atomicppm or more. On the other hand, when the oxygen residual amount is toomuch, it is supposed that glass defects such as non-bridging oxygen holecenter (NBOHC, ≡Si—O.) easily occur, and thus it is preferred that theconcentration of oxygen molecule contained in glass be 200 mol·ppb orless.

REFERENCE SIGNS LIST

-   -   1 Optical fiber preform    -   10 Core preform (Third core rod)    -   11 First core portion    -   12 Second core portion    -   13 Third core portion    -   20 Cladding portion

1. A method of manufacturing an optical fiber preform which comprises afirst core portion including an alkali metal element, a second coreportion surrounding the first core portion, a third core portionsurrounding the second core portion, and a cladding portion surroundingthe third core portion and further having a refractive index lower thaneach refractive index of the first to third core portions, the methodcomprising: a first doping step of doping an alkali metal element intoan inner surface of a first glass pipe with a mean chlorineconcentration of 10 atomic ppm or more and 600 atomic ppm or less; afirst collapse step of collapsing the first glass pipe after the firstdoping step by heating, thereby producing a first intermediate rod fromthe first glass pipe; a first diameter-reduction step comprisingremoving an outer peripheral portion of the first intermediate rod toproduce a first core rod constituting a part of the first core portion,the first core rod having a diameter smaller than a diameter of thefirst intermediate rod; a second doping step of doping an alkali metalelement into an inner surface of a second glass pipe with the meanchlorine concentration of 10 atomic ppm or more and 600 atomic ppm orless; a second collapse step of integrating the first core rod and thesecond glass pipe by heating while the first core rod is inserted intothe second glass pipe after the second doping step, thereby producing asecond intermediate rod including a first glass region to be the firstcore portion and a second glass region to be the second core portion,the first glass region having an alkali metal element concentration of100 atomic ppm or more, the second glass region having an alkali metalelement concentration of 10 atomic ppm or less; a first build-up step ofadding a third glass region to be the third core portion onto an outerperiphery of a second core rod formed from the whole or part of thesecond intermediate rod to obtain a third intermediate rod including thesecond core rod, the third glass region having an alkali metal elementconcentration of 10 atomic ppm or less and a chlorine concentration of2,000 atomic ppm or more and 15,000 atomic ppm or less; and a secondbuild-up step of adding a fourth glass region to be the cladding portiononto an outer periphery of a third core rod formed from the whole orpart of the third intermediate rod to obtain the optical fiber preformincluding the third core rod.
 2. The method of manufacturing an opticalfiber preform according to claim 1, wherein the first collapse stepcomprises introducing oxygen gas in the inside of the first glass pipein a reduced pressure condition.
 3. The method of manufacturing anoptical fiber preform according to claim 1, wherein the second collapsestep comprising introducing oxygen gas in the inside of the second glasspipe in a reduced pressure condition.
 4. The method of manufacturing anoptical fiber preform according to claim 1, the method furthercomprising a second diameter-reduction step carried out after the secondcollapse step, wherein the second diameter-reduction step comprisesremoving an outer peripheral portion of the second intermediate rod toproduce the second core rod having a diameter smaller than a diameter ofthe second intermediate rod.
 5. The method of manufacturing an opticalfiber preform according to claim 1, the method further comprising athird diameter-reduction step carried out after the first build-up step,wherein the third diameter-reduction step comprises removing an outerperipheral portion of the third intermediate rod to produce the thirdcore rod having a diameter smaller than a diameter of the thirdintermediate rod.
 6. The method of manufacturing an optical fiberpreform according to claim 1, wherein a concentration of OH groupcontained in the third core rod is 0.01 wt·ppm or less.
 7. The method ofmanufacturing an optical fiber preform according to claim 1, wherein thealkali metal element is potassium.
 8. The method of manufacturing anoptical fiber preform according to claim 1, wherein the third core rodincludes a region containing the alkali metal element having aconcentration of 100 atomic ppm or more, and in a part or the whole ofthe region an oxygen molecule concentration is 30 mol·ppb or more and200 mol·ppb or less.
 9. An optical fiber preform, comprising: a corepreform including: a first core portion with a maximum alkali metalelement concentration of 500 atomic ppm or more and 5,000 atomic ppm orless, the first core portion having a profile of an alkali metal elementconcentration along a radial direction of the first core portion, theprofile having at least two local maxima points, which are separatedfrom the center of the cross section of the first core portion, a secondcore portion surrounding the first core portion, the second core portionhaving the alkali metal element concentration of 10 atomic ppm or less,and a third core portion surrounding the second core portion, the thirdcore portion having the alkali metal element concentration of 10 atomicppm or less and the chlorine concentration of 2,000 atomic ppm or moreand 15,000 atomic ppm or less, wherein the core preform has a meanalkali metal element concentration of 7 atomic ppm or more and 70 atomicppm or less, and a cladding portion surrounding the core preform, thecladding portion having a refractive index lower than each refractiveindex of the first to third core portions.
 10. The optical fiber preformaccording to claim 9, wherein the alkali metal element is potassium. 11.The optical fiber preform according to claim 9, wherein the chlorineconcentration of the first core portion and second core portion is 10atomic ppm or more and 600 atomic ppm or less.
 12. The optical fiberpreform according to claim 9, wherein the core preform includes a regioncontaining the alkali metal element having a concentration of 100 atomicppm or more, and in a part or the whole of the region an oxygen moleculeconcentration is 30 mol·ppb or more and 200 mol·ppb or less.
 13. Theoptical fiber preform according to claim 9, wherein a mean concentrationof OH group in the core preform is 0.01 wt·ppm or less.